1. Field of the Invention
The present invention relates to an improvement in the technique for obtaining metallic iron by heating and reducing an iron oxide source such as iron ore by carbonaceous reducing agents such as coke, and a method improved so as to efficiently reducing iron oxide to metallic iron by simple treatment, to efficiently separate produced metallic iron from a slag forming component mixed in iron ore or the like as a gangue component, to produce metallic iron particles of high purity with high yield.
2. Description of the Related Art
Recently, many studies have been progressed with respect to a direct iron producing method for forming a raw material mixture containing an iron oxide source (such as iron ore) and carbonaceous reducing agents (such as coke), heating the former to thereby reduce iron oxide in the iron oxide source by the carbonaceous reducing agents, and separating produced metallic iron from a by-produced slag component to produce metallic iron.
The present inventors have also progressed the study on the direct iron producing method of this kind since long ago, and developed the following method as a result of the study, and further progressed study.
This method comprises, in producing metallic iron by heating and reducing a compact containing carbonaceous reducing agents and iron oxide, reduced iron oxide in solid state by heating to thereby produce and grow a metallic iron shell, continuing solid reducing till iron oxide is not substantially present inside, further continuing heating to flow out the produced slag from the metallic iron shell, and afterward separating metallic iron from slag.
In carrying out the above method, a part of the metallic iron shell may be molten to thereby flow out the molten from the metallic iron shell. At this time, for melting a part of or the whole metallic iron shell, carbon resulting from carbonaceous reducing agents which is present inside the metallic iron shell may be dissolved (solution) in metallic iron (this phenomenon is sometimes called xe2x80x9ccarburizingxe2x80x9d) to thereby lower a melting point of the metallic iron shell.
Metallic iron of high purity obtained by the above method and the produced slag are cooled and solidified to crush the slag and the solidified metallic iron particles are subjected to classification by magnetic separation or a sieve, or metallic iron is separated from slag by heating and melting and due to a difference in specific gravity to thereby enable obtaining a material having high purity in excess of 95 mass %, or in excess of 98 mass %. Moreover, the disclosed invention provides a method for proceeding reducing of iron oxide in solid state, which can reduce molten FeO amount in the produced slag as less as possible, in which erosion and/or corrosion of refractories of a processing furnace caused by molten FeO is hard to occur, and which is expected that the above method be realized as a practical application from a viewpoint of maintenance of equipment.
Among the above methods, the method for cooling and solidifying the produced metallic iron and the produced slag, crushing the produced slag and thereafter obtaining metallic iron particles by magnetic separation or the sieve seems to be suitable for the application to an industrial scale as compared with a method for separating them by a difference in specific gravity after melting. That is, in the melting and separating method, it is necessary to heat at high temperature for melting, because of which a great heat energy is required, and in addition, when both are separated, a part of molten iron is entrained in molten slag at an interface to possibly lower yield of metallic iron. On the other hand, in the method for obtaining metallic iron particles by crushing, magnetic separation or a sieve, heat energy is unnecessary, and in addition, design of a continuous separating system according to a scale of iron-manufacturing equipment are easy, and iron loss can be also minimized.
The above disclosed invention has stressed that in the heating and reducing step, the metallic iron shell is produced, and a high-degree reducing atmosphere is formed within the shell whereby metallizing is progressed efficiently. However, according to later study, it has been confirmed that when the neighborhood of a raw material compact is kept in a higher reducing atmosphere by a large amount of CO gas generated by combustion of carbonaceous reducing material included in the raw material compact, such a metallic iron shell is not always necessary.
On the other hand, with respect to the method for controlling a producing slag composition to accelerate separation of metallic iron when the direct iron manufacturing method as described above, several methods have been proposed.
For example,, there is a method for using ironmaking dust as a iron oxide source, mixing it with carbonaceous material (carbonaceous reducing agents) and additional material (slag forming agent), controlling a producing slag composition in a range of 1.4 to 1.6 at CaO/SiO2 ratio (basicity), subjecting it to heating and reducing at 1250 to 1350xc2x0 C. to produce metallic iron, and separating metallic iron particles from slag having a low melting point containing FeO.
However, this method is a method for using ironmaking dust as an iron oxide source, and control of basicity used in this method is at the time of initial raw material preparation. In this method, there is no recognition on the behavior of slag produced during heating and reducing, that is, behavior when the product slag turns to a molten state in a solid-liquid co-existing state affects on acceleration of separation of produced metallic iron. Further, In this method, the slag having a low melting point containing FeO is used to accelerate separation of metallic iron, but the method using the molten slag containing FeO has many problems noted below from a viewpoint of actual operation:
1) the molten slag containing molten FeO greatly damages fire brick of a hearth;
2) molten FeO comes in contact with carbonaceous material to produce reducing reaction, which reaction is endothermic reaction to make temperature control difficult; and
3) since metallic iron produced by contact reaction between molten FeO in slag and carbonaceous material is scattered in a fine granular form in slag, work for cooling and solidifying it together with slag after recovering becomes extremely complicated.
Accordingly, it is desirable to recover metallic iron efficiently while suppressing the production of molten FeO in the by-product slag.
There is a further method for, in charging a raw material mixture containing fine iron ore and solid reducing agents onto a moving hearth and heating and reducing it to produce metallic iron, previously laying fine solid reducing agents on the hearth, progressing heating and reducing in a state that the raw material iron ore is placed in a small-section on it not to contact directly with the hearth, and melting reduced iron at least once on the hearth. According to this method, the reason why xe2x80x9cplaced in a small sectionxe2x80x9d as termed herein is to prevent the molten substance containing produced metallic iron by heating and reducing and by-product slag from being fused or stuck on the hearth surface to corrode the hearth. However, for carrying out the method as described above, not only complicated equipment is necessary in order to form the small section or to charge raw material into the small section but also a large quantity of fine solid reducing substances are necessary, which method is not to be considered as a practical method in view of efficiency of raw material. Moreover, in this method, formation of the small section rather accelerates fusion and sticking of the molten substance on the hearth surface to disturb the discharge of produced substances.
Further, the above invention takes measures for preventing the damage on the assumption that the molten substance produced by heating and reducing could give the damage to the hearth refractor. However, it is rather important, in terms of actual operation, to reduce the great amount of fine solid reducing agents. Further, also from a viewpoint of economy and design of equipment, it is desired that the technique be established to reduce the damage of hearth refractory by the slag itself, and so that even after cooling and solidifying, slag or metallic iron does not stick on the hearth surface.
There is another method for controlling basicity of a slag component in raw material to a range of 0.4 to 1.3, controlling not less than ⅓ of time required for heating and reducing on the hearth to a temperature range of 1200 to 1350xc2x0 C. to make the reduction degree of iron oxide 40 to 80%, and subsequently, melting a reduced substance.
The control of basicity employed in this method is carried out by computation when raw material is prepared, and basicity is determined on the assumption that all the slag components in raw material are molten. However, whether or not all the slag components are molten changes depending on the operating conditions (particularly, temperature). Further, there is no pursued how dynamic behavior from the start of melting of slag to melting of the whole through the solid and liquid coexisting state affects on the separating condition of produced metallic iron and the erosion and/or corrosion of the hearth refractory. There is not recognized at all that the liquid fraction of the solid and liquid coexisting phase is controlled or melting of metallic iron is accelerated thereby.
With respect to the technique for heating, reducing and melting a mixture containing iron oxide source and carbonaceous reducing agents to manufacture metallic iron as described above, many proposals have been made. Recent problems pointed out in connection with the related art including the above matter are arranged and summarized as follows:
1) In heating, reducing and melting a mixture containing an iron oxide source carbonaceous reducing agents to manufacture metallic iron, it is necessary to establish the technique capable of melting solid produced metallic iron by reducing efficiently at a lower temperature, successfully separating it from the by-product slag, and separating and recovering metallic iron of high purity at a lower temperature and with high efficiency.
2) For achieving the aforementioned technique, it is desired that carburizing solid metallic iron produced by heating and reducing is accelerated to melt metallic iron at a lower temperature and efficiently, and successful separation from the by-product slag can be made to manufacture metallic iron of high Fe purity efficiently. Here, to enable controlling concentration of carbon property, which is an important factor of placing product metallic iron for practical use, is very advantageous of being used practically as steel-making material for electric furnaces or the like.
3) In the related art, some methods for controlling the slag component in raw material by basicity or the like have been proposed as mentioned above. They are proposed for the final product slag. However, if metallic iron can be molten and separated efficiently with the required minimum slag amount without melting the whole slag by-produced in the heating and reducing step, a bad influence on refractories of the hearth can be further decreased, and in addition, being advantageous in terms of heat efficiency and maintenance of equipment.
4) It is well known that molten FeO in slag greatly affects on the damage of the hearth refractory. For suppressing such damage, it is desirable to reduce the amount of molten FeO in the produced slag as less as possible. If reduction in the amount of molten FeO is realized, the damage of the hearth is relieved considerably accordingly to enable relieving particular mechanical or operational load required for the protection of the hearth.
The present invention has been accomplished paying attention to the problems as noted above. It is an object of the invention to provide a method capable of overcoming the problems as mentioned in 1) to 4) above completely to manufacture metallic iron of high Fe purity efficiently under the stable operation with suppressing the refractory damage of the hearth as less as possible.
The method for producing metallic iron according to the present invention provided is a method for heating, reducing and melting a raw material mixture containing carbonaceous reducing agents and an iron oxide-contained substance to manufacture metallic iron, comprising: controlling a liquid fraction in a solid and liquid coexisting phase of a producing slag containing a multi-component system gangue component to thereby accelerate melting of solid metallic iron produced, and efficiently separating metallic iron from by-produced slag at a lower operating temperature and with less time to manufacture metallic iron of high purity.
In carrying out this method, a liquid fraction in a solid and liquid coexisting phase of a producing slag containing a multi-component system gangue component is controlled, and the carbonaceous reducing agents are introduced into the slag in the liquid and solid state to accelerate carburizing relative to solid metallic iron whereby a melting temperature of the reduced iron, thus progressing melting of the reduced iron. It is desirable for effectively realizing such an operation as described to regulate the amount of carbonaceous reducing agents compounded in a raw material mixture so that concentration of carbon in metallic iron is 0.5 to 4.3 mass %, and to control so that a melting temperature of metallic iron subjected to carburizing is 1147 to 1500xc2x0 C.
Further, the liquid fraction of the product slag can be regulated by mixing raw materials when the raw material is prepared. More specifically, there is a method in which when the raw material mixture is prepared, a relation between a temperature of the producing slag and the liquid fraction is obtained in advance from a composition of the raw material mixture, and the other slag component is added to the raw material component whereby the optimum slag liquid fraction is obtained in a predetermined operating temperature level, or there is a further method in which the liquid fraction is controlled by a target melting starting temperature after raw material has been reduced.
For achieving the object of the present invention more effectively, it is desired that the liquid fraction of the slag at the time of carburizing and melting be controlled to a range of 50 to 100 mass %, more preferably, a range of 70 to 100 mass %. As the raw material mixture, a raw material mixture may be used without modification or used in a suitably pressed state. However, more preferably, it is desired that a mixture is agglomerated into generally spherical, briquette-like or pellet-like form for heating and reducing.
According to the present invention, the liquid fraction of the product slag is regulated to thereby enable suitably controlling the carburizing amount to solid metallic iron to be produced and as a result, the carbon concentration of product metallic iron can be also controlled. Further, according to the present invention, metallic iron condensed by carburizing and melting is cooled and solidified to thereby enable obtaining metallic iron particles. The metallic iron particles can be separated from the cooled and coagulated feeble by-produced slag by a sieve or magnetic separation, and metallic iron particles can be easily recovered.
Further, the present invention is characterized in that metallic iron is manufactured efficiently preferably in a particle form. As the secondary effect resulting from the control of the liquid fraction of the producing slag employed in the manufacturing method, the by-produced slag can be separated and recovered in a granular or particle form with a relatively uniform size distribution. More specifically, the slag from gangue minerals in raw material is cooled after heating, reducing and melting, which is classified into a grassy granular slag produced from a liquid phase of a solid and liquid coexisting phase and a granular powder slag produced from a solid phase of a solid and liquid coexisting phase for separation and recovery. Then, the granular slag of uniform size and the granular powder slag can be obtained simply.
Further, according to the present invention, the molten FeO amount in the product slag can be reduced to not more than 50 mass %, preferably, to 0% substantially to thereby suppress the erosion/corrosion of the hearth refractory caused by mixing of a large amount of molten FeO into the slag as less as possible. Particularly, when the raw material mixture is heated and reduced, if a heating speed of the raw material mixture is raised to not less than 300xc2x0 C./minute, the molten FeO amount in the producing slag can be effectively reduced, which is preferable.
As described above, the present invention has the greatest characteristics in that in heating, reducing and melting a raw material compact containing an iron oxide-contained material (hereinafter sometimes referred to as iron ore or the like) such as iron ore and iron oxide or its partial reduced substance and carbonaceous reducing agents such as coke and coal (hereinafter sometimes referred to as carbonaceous material) to manufacture metallic iron, a liquid fraction in a solid and liquid coexisting phase of a by-produced slag containing a multi-component system gangue component produced resulting from iron ore or the like is controlled to thereby efficiently progress carburizing of produced metallic iron, whereby a melting point of metallic iron is lowered quickly to thereby accelerate melting (hereinafter sometimes referred to as xe2x80x9cmelt-downxe2x80x9d).
As described previously, in the related art, a method has been proposed for regulating basicity or the like of by-produced slag in view of a melting point when the gangue component resulting from iron ore or the like is molten wholly. On the other hand, in the present invention, the by-produced slag is not always molten wholly, but a new conception, a liquid fraction in a solid and liquid coexisting phase of the by-produced slag is introduced to effect control, and the invention has been accomplished on the basis of new knowledge that the liquid fraction is in a close relation with the melt-down of metallic iron. That is, in the present invention, the liquid fraction is controlled property whereby the solid metallic iron produced by heating and reducing can be lowered in a melting point by progressing carburizing at a low operating temperature to thereby enable melting-down of metallic iron at a lower temperature. Thereby, separation from the by-produced slag can be progressed efficiently at a low temperature, and concentration of carbon greatly influencing on quality of product metallic iron can be also controlled.